A simple, sensitive and reproducible whole blood microculture assay has been developed for quantitatively evaluating lymphocyte reactivity to a variety of nonspecific (mitogenic) and specific (antigenic) stimulants. This assay employs heparinized whole blood, serum-free culture medium, microtiter plates and an automated sample harvester. When compared to other leukocyte assays, the major advantages realized by this assay include: (a) the utilization of fewer lymphocytes per micro-culture, thus reducing the amount of blood required per test, while increasing the number of test agents and replicate cultures which can be employed in any given experiment, (b) the conservation of test agents, some of which are either expensive or difficult to prepare in large quantities, (c) the elimination of lymphocyte isolation and purification procedures which may disrupt the relative proportions of T-cells, B-cells and antigen-processing accessary cells, and (d) the application of an automated harvester which expedites procedures required for measuring the cellular incorporation of radiolabeled compounds. In addition to monitoring the immune status of different individuals, this assay may also be useful for screening and identifying agents such as antibodies, drugs, hormones, lymphokines and other materials which may be of therapeutic value. Studies currently in progress are attempting to define the utility of this assay for defining other parameters of lymphocyte function. BIBLIOGRAPHIC REFERENCES: Pauly, J. L., Minowada, J., Han, T. and Moore, G.E.: Disparity of mixed lymphocyte reactivity to cultured cells of human T and B lymphoid lines. J. Natl. Cancer Inst. 54: 557-562, 1975. Han, T., Pauly, J.L. and Minowada, J.: Disparity in the production of lymphoblastogenesis inhibition factor by cultured human B and T lymphoid cells. Clin. Exp. Immunol. 20: 73-81, 1975.